Distributed temperatures sensors are advantageously employed whenever the temperature is to be determined in different points of a relatively wide ambient, as they eliminate the drawbacks inherent with collecting an transferring information from a great number of individual sensors. Optical fiber sensors in particular have the advantage of being resistent to high temperatures and, if the fiber is used also as transmission means, they also allow remote monitoring and hence can be used to detect temperature in regions which are not accessible or are accessible only with difficulty and/or risk.
A number of temperature sensors employing an optical fiber as sensing element are known in the art.
For instance, U.S. Pat. No. 4,576,485 discloses a sensor in which the fiber is capable of internally generating thermal radiation in response to the temperature of the monitored region and has constant and discrete absorption values for each wavelength of the radiation spectrum; temperatures values along the fiber are obtained by measuring the power associated with a plurality of spectral bands into which the fiber output radiation is subdivided.
This system has a number of drawbacks: it requires the use of special fibers (fibers for infrared) which are not yet commercially available; and it is based on the detection of absorption peaks of an internally generated radiation and hence, with certain wavelengths, the fiber output power may be insufficient for detection, so that the temperature data may be imprecise.
Another distributed temperature sensor is disclosed by M. C. Farries, M. E. Fermann, R. I. Laming, S. B. Poole, D. N. Payne and A. P. Leach in the article "Distributed temperature sensor using N.sup.3+ doped optical fiber", Electronics Letters, Vol. 22, No. 8, Apr. 10, 1986. In this known sensor, a radiation is launched into the fiber and the attenuation of the backscattered radiation is measured at the wavelengths corresponding to the absorption peaks due to Nd ions, such attenuation being dependent on the doping-ions concentration and on temperature. The measurements on the backscattered radiation are effected by an optical time domain reflectometer (OTDR) which allows the attenuation information to be associated with a position along the fiber. Thanks to the use of a radiation launched from the outside into the fibre, this system is free from the sensitivity problems of the sensor of the identified U.S. patent, provided the power source is sufficiently high. Yet it still requires the use of a special fiber and the precision of the information obtained depends on the precise knowledge of the dopant concentration. Moreover attenuation measures are effected, so that the resultant also depend on the precise knowledge of the source power.